1. Field of the invention
This invention pertains to data communications and particularly to data communications on a wireline such as one employed in an oil or gas well borehole application.
2. Description of the Prior Art
It is common in an oil or gas well borehole application to transmit and receive electrical digital data and control signals between surface electronics and downhole electronics package via a wireline of one or more conductors connecting the two. Such signals are typically used to remotely control the functions of various downhole devices such as sensors for detecting borehole parameters as well as tools and devices for performing functional operations in the borehole such as setting equipment or operating testers, motors, directional drilling equipment or the like, which may be operable in stages and in any event requiring a plurality of differing control signals at different times. Likewise, it is desirable to transmit information indicative of the operation of the downhole devices or parameters detected or measured downhole, to the surface over the same conductor path. It is customary in such downhole operations to utilize a sheathed or armored cable which includes either a single conductor or multiple conductors. A single conductor armored cable typically includes a single insulated conductor as a core, and a protective conductive sheathing surrounds the insulated core. The core and sheathing form an electrical circuit path for transmitting electrical power and data. The standard multi-conductor armored cable is a 7-conductor armored cable used for multiple channel tools. Such so called single conductor wireline cables, or similarly constructed multi-conductor cables, are almost exclusively used to operate downhole electrical devices because of a variety of reasons associated with the space limited and rigorous environment of a borehole. In such oil and gas borehole operations, a borehole depth of many thousands of feet is not uncommon. In communicating between the surface and downhole in a borehole over a wireline cable, control signals and data signals are normally converted to digital signals transmitted by a transmitter at rates up to a maximum of 20 Kbits/second. A receiver on the other end of the cable receives the signals, and a processor decodes the signals for further use.
The transmission and receiver scheme described above operates well when the rate of transmission does not exceed about 20 Kbits/second or the wireline is relatively short. However, the wireline transmission medium does cause a problem when the transmission is over a relatively long length or as the data rate increases. That is, the detection and distinguishing of the two voltage levels associated with the digital signal is impaired by distortions caused by the medium. Distortions become more acute for faster bit rates, where the periods at each of the two voltage levels are very short. For example, the frequency characteristic of a typical single conductor wireline used for downhole application has a loss of about −20 db at 5.6 Khz for a 30,000 foot length. At higher frequencies, the loss is significantly greater.
Often, multi-conductor cables are used when multiple channels to several sensors are used. The most commonly used cable today is a 7-conductor armored logging cable. For comparison purposes, a cable of at least 30,000 feet in length wherein the cable is a 7-conductor cable provided within an armored logging cable having a nominal size of 7/16 inches has a frequency bandwidth of 90 to 270 Khz. Bandwidth is defined as the frequency at which an input signal is attenuated to the point where the signal cannot be effectively recovered by the receiving device. Typically, and for the purposes of this disclosure, the attenuation is −60 db.
Today, while the wells become deeper, the measuring devices have also become more complex. That is, they provide data at a much greater rate. Moreover, the advent of digital computers installed at the well head measuring equipment has enabled the handling of greater volumes of data in a more effective fashion. All of this has occurred simultaneously increasing the requirements on the logging cable. The cables have become more complex i.e., they have added conductors, and the band pass requirements for the conductors have been increased. Still, the cables used today are unable to provide bandwidth in deep wells matching the transmission capabilities of the instrumentation.
There are several factors affecting the bandwidth of a particular cable configuration including resistance (R), capacitance (C), inductance (L) and conductance (or leakage.) Typically gains to be achieved in inductance and conductance are small since these factors are negligible. The most straightforward correction for high resistance of a cable, which is proportional to the diameter cable conductors, is to have larger diameter cables. This correction is opposed by the need to balance cable size with borehole parameters. Parameters such as borehole diameter and fluid pressure lead designers to smaller diameter cables. Capacitance of logging cables has been minimized, thereby increasing bandwidth, by adding conductors or by using a coaxial cable. As discussed earlier, the coaxial cable is used by referencing a signal to the shield (or armor.) Although capacitance is improved, the capacitances of typical coaxial and multi-conductor cables are still around 40 to 60 pF/ft.
Surface communication cables often utilize twisted pairs of conductors to increase bandwidth over single conductor transmission cables. The term twisted pair conductor, as used herein is defined as two electrically-conductive wires, which are electrically insulated from each other and twisted about each other at a given non-zero twist rate. Twisted pair conductors have heretofore been used in downhole applications only with the aide of supporting clamps or structures. One example of a clamped system is U.S. Pat. No. 6,206,133 for “Clamped receiver array using tubing conveyed packer elements”. Another example is U.S. Pat. No. 6,580,751 to Gardner, et al. for “High speed downhole communications network having point to multi-point orthogonal frequency division multiplexing.” The '133 patent describes a geophone array permanently or semi-permanently installed within a well borehole and communicating with a surface computer over twisted wire pairs. Such arrays as described in the '133 and '751 patents are not wireline systems and are unsuitable for self-supporting wireline logging in the drilling phase due to the need quickly insert the wireline data logger into a well borehole, take measurements and then remove the wireline all during a tripping cycle of the drill string.
One problem with implementing twisted pair conductors in a self-supporting wireline is stress induced at each twist crossing point causes conductor deformation or failure at the crossing point when high tensile loads are supplied. Therefore, prior wireline systems are typically designed to a standard wireline cable using single conductors or systems are designed with complicated clamping measures to secure and support the cable during use. An example of a standard wireline cable is described in U.S. Pat. No. 3,259,675 to Bowers for “Method of Manufacturing Armored Cables”. The '675 patent describes a typical 7-conductor wireline cable, which includes a central conductor surrounded by six outer insulated conductors. While the outer conductors are helically wound, they are not twisted pairs as the term is known to those in the art and as the term is used herein.
To address some of the deficiencies described above, the present invention provides a load bearing cable having improved bandwidth and lower capacitance per foot for use in wireline applications. This invention also provides a multi-conductor load bearing cable used in a single conductor mode with lower capacitance than the typical single conductor cable used today.
Although increasing the bandwidth of a cable is necessary to improve data rate transmission, it should also be appreciated that the efficient use of the bandwidth is also required. As discussed earlier, instruments now have the capability to transmit data at rates far beyond cable capabilities. Methods of encoding data for transmission used in the telecommunication industry include Quadrature Amplitude Modulation (QAM), Carrierless Amplitude and Phase (CAP) modulation, and Discrete Multi-Tones (DMT) modulation. CAP is a modified QAM method, and DMT is the method in digital subscriber line (DSL) applications currently marketed mainly as an enhancement to internet connections. At this time, the well logging community has not taken advantage of the state of the art encoding methods. The primary driver being that the cables in current use cannot provide the bandwidth necessary to utilize these encoding methods efficiently.
To meet the demand for higher data rates, the present invention provides a system utilizing telecommunication data encoding methodologies in conjunction with a load bearing data cable having enhanced bandwidth to increase transmission data rate.
This invention also provides a method of well logging data transmission having a higher data rate.
In general, the present invention provides a logging data transmission method and apparatus. The apparatus includes a logging cable having improved bandwidth characteristics.
In one embodiment, a suspended well logging data cable comprises an outer protective sheath and a tensile load carrier positioned within the outer protective sheath, the tensile load carrier enabling the self support of the well logging cable while suspended in a well borehole. At least one twisted pair of signal conductors is positioned within the outer protective sheath, each of the conductors being separately insulated, the at least one twisted pair of signal conductors having a twist rate of at least ⅙ twist per inch.
In another embodiment, a cable is provided having at most seven twisted pairs of conductors disposed around a center conductor and operating in a single conductor mode or in differential mode.
In one aspect, the wireline logging cable includes at most seven twisted pair signal conductors.
In one embodiment a system having an improved data transmission rate is provided comprising a downhole well data sensor and a downhole data transmitter such as a modem and an encoding method of QAM, CAP or DMT. Included in the system is a surface data receiver complementary to the downhole transmitter. A well logging cable linking the transmitter and the receiver includes an outer protective sheath, at least one twisted pair of signal conductors positioned within the outer protective sheath, each of the conductors being separately insulated, the at least one twisted pair of signal conductors having a twist rate of at least ⅙ twist per inch, an insulation sheath surrounding the at least one twisted pair of signal conductors, and a tensile load carrier surrounding the insulation sheath, the tensile load carrier comprising a sheath of tensile load carrying filaments enabling self support of the well logging cable, the downhole well data logger and the downhole transmitter while suspended in a well borehole.
In one embodiment a method of transmitting data from a well borehole to a surface location comprises transmitting the signal with a downhole data transmitter and conveying the signal on a suspended well logging cable linking the transmitter and to a surface receiver, the well logging the cable having at least one twisted pair of signal conductors positioned within an outer protective sheath, each of the conductors being separately insulated, the at least one twisted pair of signal conductors having a twist rate of at least ⅙ twist per inch, an insulation sheath surrounding the twisted pair of conductors and a tensile load carrier surrounding the insulation sheath, the load carrier comprising a sheath of tensile load carrying filaments enabling self support of the well logging cable, the downhole well data logger and the downhole transmitter while suspended in the well borehole.
For detailed understanding of the present invention, references should be made to the following detailed description of the preferred embodiment, taken in conjunction with the accompanying drawings, in which like elements have been given like numerals and wherein:
The cable 100 includes a tensile load bearing tubing 112 comprising an inner layer 114 and an outer layer 116 of wires. The inner layer of wires 114 is a plurality of stranded structural steel wires with 0.025″ (0.64 mm) total outer diameter helically wound around the elastomer bedding 110. The outer layer 116 is a plurality of stranded structural steel wires with 0.0345″ (0.88 mm) total outer diameter helically wound around the inner layer 114. An outer protective sheath 118 may be used to protect the cable against abrasions caused by running the cable in and out of the borehole. The twisted pairs of signal conductors and the tensile load carriers will thus be within the outer protective sheath 118.
The overall outer diameter of a cable built to these dimensions would be 0.025″ (6.35 mm). The relationship between resistance and diameter of a conductor is inversely proportional and the load bearing capability is directly proportional to the diameter. These relationships would normally lead one to larger cable designs. However, the overall diameter of a cable should be minimized in a downhole application, because the pressure of the fluid in the well may force a cable out of the well if the diameter is too large.
Referring now to
The most useful capacitance to know is the effective capacitance per foot (Ceff) of the cable. This is the effective capacitance between the conductors 102 and 104. To determine Ceff, equations are used that require measured values between the conductors 102 and 104 (designated as C12m) and between each conductor and the armor 112 (designated as C13m and C23m respectively.) The computation is initiated with an experienced based empirical value of 1 F for the same parameters, C12, C13 and C23. To determine the actual C12 or Ceff, equations are then set up as follows:
The equations are then iteratively solved for the correct values of C12, C13, and C23 yielding:
C12=2.999×10-11 F/m;
C13=8.999×10-11 F/m; and
C23=8.999×10-11 F/m.
Therefore, since 1 m=3.28084 ft, the Ceff of C12 for the cable described is actually 9.144 pF/ft. Compare this to the typical cable values of 40-60 pF/ft as stated above. The capacitance and conductor configuration of a cable according to the present invention results in a bandwidth of about 350 KHz.
There are two modes of operation or configuration modes useful for the twisted pair cable described above. These are the single conductor mode and the twisted pair or differential mode. In the single conductor mode, the ends of the conductors 102 and 104 are tied together electrically. A signal transmitted on the cable is then sensed with reference made to the armor 112. In the differential mode, the conductors 102 and 104 are each used independently for signal transmission, and the signal is sensed as a differential between the conductors 102 and 104. The bandwidth of either configuration is larger than the bandwidth of current single conductor load bearing cables used in well logging systems.
In this configuration, center conductor 302 is shown as a single conductor. However, the intent is not to exclude the use of a twisted pair for the center conductor. Also, the preferable mode for the twisted pair wires is the single conductor mode where the ends are electrically connected, but the differential mode may be preferable in a particular application. As known in the art, any conductor may carry both data and power simultaneously.
Referring to
The twisted pair signal conductors of
At the surface the cable is carried by a sheave and winch assembly 414, and the end of the cable 412 is connected to a surface control unit 416 comprising a surface modem 418, a processor 420, an output/storage device 422 . The surface modem is complementary to the downhole modem 410, and the processor 420 is connected to the surface modem 418 to receive, decode and process the data transmitted to the surface. The processor 420 is also used to send commands to the instruments downhole via the modem-cable-modem connection. An output device/storage 422 such as a display screen, printer, magnetic tape, CD, or the like is connected to the processor for display and/or storage of the processed data. The output device 422 may also include a transmitter 424 for relaying the processed data to a remote location.
In operation, a well engineer or user deploys the tool 402 supported by the cable 412 in the well 404 to a desired depth using the winch and sheave mechanism 414. Commands generated by user input, algorithm, or a combination are encoded at the surface using one of the methods described above. The encoded commands are then transmitted by the modem 418 through the cable 412 to the tool 402 disposed in the well. The downhole modem 410 receives the command which is then decoded for downhole operation of the tool.
When sensors 406 are activated to sense a desired parameter, the sensed parameter is delivered to the downhole processor 408 for pre-processing or sent directly to the surface. In either case, the data is encoded using one of the methods described above and transmitted by the downhole modem 410 through the cable 412 to the surface control unit 416. At the surface, the surface modem 418 receives the data. The processor 420 decodes the signal, performs further processing of the data, and the data is then displayed on a screen, printed on a printer, stored on magnetic tape, CD, or the like. The data may also be relayed to any remote location using a transmitter 424.
The foregoing description is directed to particular embodiments of the present invention for the purpose of illustration and explanation. It will be apparent, however, to one skilled in the art that many modifications and changes to the embodiment set forth above are possible without departing from the scope of the invention and the following claims.
This application is a continuation-in-part of U.S. application Ser. No. 09/586,130 filed on Jun. 2, 2000, now abandoned, which application took priority from U.S. Provisional Application No. 60/193,098 titled “Improved Bandwidth Wireline Data Transmission System and Method” filed on Mar. 30, 2000, the entire specification of each application being hereby incorporated herein by reference.
Number | Date | Country | |
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60193098 | Mar 2000 | US |
Number | Date | Country | |
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Parent | 09586130 | Jun 2000 | US |
Child | 11086944 | Mar 2005 | US |